D.P.
I believe Excel has formulas for that kind of equation...but you need to factor in interest rates, etc.....
Have you looked on the Dave Ramsay website? He's got lots of tools and calculators there.
Here's O. I found:
http://cgi.money.cnn.com/tools/debtplanner/debtplanner.jsp
According to this calculator (for example) it will take 5 years to pay off 100K at 18% with a monthly payment of $2500. You will pay $50,108!!!